Question in regards to husbands marriage in another country

[Rachel_O]

My husband and I were married last year and recently have started immigration proceedings to adjust his status in this country. We found out that my husbands 1st marriage "in Mexico" is considered valid in the US even though Mexico considers him divorced since he hasn't been with his wife in 14 years--the "wife" has also went on with her life as well and is married to another man (by the way my husband has a 13 yr old child from this marriage).My immigration attorney isn't able to give me much info in regards to the steps that I need to take in order for my husband to get a divorce from wife #1. I know that he needs to file for the divorce BUT do we need to get our marriage annuled or is it automatically considered invalid due to him already being married? Any advise would help! Need to know how to start on getting this issue resolved.